This invention relates to a method and apparatus for freezing articles with cryogenic liquids, and more particularly, to freezing prepared foods and the like at a substantially constant temperature.
Foods having high water content cannot be satisfactorily frozen with conventional refrigeration systems utilizing freon and other conventional refrigerants. These systems have a relatively slow freezing rate which produces ice crystal growth in fruits and other foods, which ruptures the delicate cell walls and results in collapse of the food upon thawing.
With cryogenic liquids, such as liquid nitrogen, however, freezing rates can be obtained which are so fast that high water content products can be frozen whereby little or no collapse occurs upon thawing. The rate of freezing with liquid nitrogen is so rapid that the food is frozen with little or no loss of flavor. Moreover, since the water content of the frozen perishables is retained, the food is prevented from becoming dehydrated, dry and flakey.
Cryogenic freezers and systems are well known. Liquid nitrogen flash freezing of meat, poultry, seafood, bakery products, citrus fruits, prepared foods and non-food products are disclosed in U.S. Pat. No. 3,494,140. Mass freezing of conveyorized products are described in U.S. Pat. Nos. 3,345,828; 3,376,710; 3,805,538 and 3,393,532; and cryogenic flash tunnels are described in U.S. Pat. Nos. 3,287,932 and 3,871,185. U.S. Pat. Nos. 3,403,527 and 3,879,954 describe countercurrent vapor recirculation flash freezing and U.S. Pat. No. 3,238,736 illustrates a manifold for ejecting liquid nitrogen in proximity to the outlet of a flash tunnel. These are typical of disclosures of liquid nitrogen freezing apparatus. In conventional cryogenic freezers, such as those described above, the nitrogen propelled against the articles to be frozen varies in temperature along the length of the freezer.